During the previous funding cycles, our program demonstrated that donor-specific memory T cells generated during protective immune responses against environmental pathogens constitute an important barrier to tolerance induction in adults. More recently, within the context of the increasingly-appreciated heterogeneity in memory T cell populations, a modern view of alloimmunity is developing, recognizing that a patient's risk for rejection and their susceptibilities to tolerance induction are highly individualized and determined in large part by their cumulative history of episodic and sustained encounters with a diverse array of environmental pathogens. The unifying hypothesis and theme of this program is that the efficacy of tolerance induction in a given individual is a function of the specific type and frequency of environmental pathogen exposure, in that these exposures generate distinct memory profiles which are composed of populations of highly specialized subsets of variable size. Accordingly, this program will be comprised of three inter-related projects. Project I seeks to define the precise mechanisms by which tolerance can be induced in na[unreadable]ve T cells by CD28 and/or CD40 blockade, and then to define specific aspects and subsets of the memory compartment that pose the largest barrier to tolerance, including traditional CD4+ and CD8+ TCM and TEM as well as the newly described, potentially very interesting subsets of CD4+ Th1 memory cells that have been shown to have different phenotypic, functional, and homing properties. Analysis of these subsets will be the major focus of Project II, which will pursue in-depth studies of the differentiation and lineage relationships of these subsets, and will examine their relative abilities to provide help for protective B and T cell responses. Project III will test the hypothesis that the immune control of latent herpesvirus infections fosters, through the generation of viral-specific memory T cells, the maintenance of memory phenotype T cells with heterologous alloreactivity. These cells may be refractory to immunosuppression in general and to T cell depletion in particular, and to the extent that they cross-react with donor antigens, they may specifically prevent allograft acceptance. The knowledge gained through these studies will facilitate development of tolerance induction protocols that are efficacious, safe, clinically applicable, and have the potential to be tailored to fit the environmental pathogen exposure of each individual patient in order to maximize graft survival.